This invention is directed to a novel composition useful in preparation of solder mask coatings. More specifically, the invention relates to a liquid photopolymerizable composition which is capable of application by means of screen printing to a substrate, following which the composition may be converted to a dry, highly resistant coating without necessity for evaporation of solvents and/or thermal curing.
Solder mask coatings are typically used on quality printed circuit boards where molten solder, by a process known as wave soldering, is used to insure good interconnections between the components and the circuitry. In such cases, a solder mask must be extremely resistant to heat, impervious to a wide range of solvents, strongly adherent to a wide variety of metallic and non-metallic substrates, and chemically resistant to the conventional rosin-based fluxes. Furthermore, since solder masks are usually left on the circuitry as a protective coating, same must also have long term durability and excellent electrical insulating properties.
Conventional solder mask or resist coatings of the prior art typically contain a significant amount of solvent, which must be removed from the coating, and also typically are based on systems which require thermal curing. Such systems consume a great deal of energy and time, and the solvent vapors contained therein are typically pollutants.
For these reasons, over the last several years there has been increased interest in utilizing solder masks which are curable by ultraviolet radiation as opposed to the foregoing systems, the incentive for same being obvious. Recently, U.S. Pat. Nos. 4,003,877 and 4,064,287 have issued which disclose a photopolymerizable composition which is asserted to be useful as a solder mask. The photopolymerizable materials disclosed therein are screen-printable, a conventional technique for the application of solder resists or masks to a substrate, and contain from 35 to 70 percent by weight of an aryloxyalkyl acrylate monomer or prepolymer, from about 15 to about 45 percent by weight of a photopolymerizable diluent containing at least one terminal ethylenic group, and a free-radical generating system activatable by actinic radiation.
I have now found a composition, suitable for use as a solder mask, which is based on the foregoing aryloxyalkyl acrylates together with hereinafter defined diacrylate diluents and styrene and its monofunctional analogs.